There have been proposed a number of techniques aiming at elevating the capacity of absorbing bodily fluid of paper diapers, sanitary napkins and incontinence pads and thus these products have been improved thereby. Most of these improvements include increase in absorption rate, prevention of the back-flow of the bodily fluid from the surface of an absorbent, prevention of leakage and relief of stickiness to the skin.
Regarding a material for an absorbent article, for example, it has been proposed to elevate the absorption capacity and to prevent the post-absorption back-flow by using a high-absorbent polymer, whereby a liquid is absorbed and retained by taking advantage of a physicochemical action, i.e., ion osmotic pressure, as a substitute for hydrophilic absorbent papers or pulps whereby a liquid is absorbed through a physical microspace. In fact, this proposal contributes to the improvement in the absorption capacity. Thus absorbents comprising a combination of a pulp and a high-absorbent polymer are employed in almost all absorbent articles at present.
However, these absorbent articles are still insufficient in the prevention of leakage, i.e., suggested the major dissatisfaction with absorbent articles resides in leakage. Also, it is pointed out that conventional absorbent articles have only a limited rate of absorbing bodily fluids and it is therefore unavoidable to use a high-absorbent polymer together with, for example, pulp having a high absorption rate, which also proves the above-mentioned disadvantage.
As one of the reasons for the unsatisfactory effect of preventing leakage of conventional absorbent articles, it may be mentioned that when pulp forms a flexible fluff absorbent layer as an absorbent, it absorbs blood on the spot and thus exhibits only poor liquid diffusion characteristics for utilizing the whole absorbent efficiently. Although the fluff pulp shows a recovery from compression and bending at a certain level in a dry state, it suffers from a serious decrease in strength in a wet state and thus scarcely shows any recovery. When stress is applied to wet pulp, therefore, the pulp undergoes compressive deformation (hereinafter referred to as "twisting"), which considerably reduces the absorption space thereof. When the absorbed bodily fluid is returned due to the twisting, the decrease in the pulp fiber space accompanying the twisting causes an increase in the transfer resistance of the bodily fluid toward the polymer. As a result, the absorption efficiency of the polymer is lowered and the re-absorption rate of the twisted absorbent as whole is also seriously lowered, which often results in leakage.
As a technique for improving the liquid diffusion characteristics of the fluff pulp, there has been reported it is possible to produce a high-density pulp by compression to give thereby improved diffusion characteristics. However, this technique not only fails to fundamentally solve the problem of twisting of pulp but also worsens the absorption efficiency of the polymer, since the inter-fiber distance of the pulp is shortened and thus the transfer resistance of bodily fluid toward the polymer is significantly elevated. Thus, conventional absorbents comprising fluff pulp are still insufficient in absorption capacity and prevention of leakage and cannot provide both a high absorption rate and good liquid diffusion characteristics at the same time.
Most of the conventional absorbent papers are those obtained by paper-sheeting natural pulp by the wet method. When a common absorbent paper obtained by sheeting natural pulp is dehydrated and dried in the dehydration/wet-pressing/drying steps during the paper-sheeting process, a strong restraint force occurs due to the interfacial tension of water and hydrogen bonds between fibers upon dehydration/drying. This restraint force makes the inter-fiber distance of the pulp very tight. As a result, the absorbent paper obtained from this pulp exhibits only a very slow absorption/permeation of a liquid. Furthermore, the pulp fiber space, by which the liquid is substantially absorbed, is extremely reduced.
Attempts have been made to elevate the bulkiness of absorbent paper by, for example, creping or embossing. However, these processing techniques can scarcely enlarge the absorption space essentially consisting of pulp fibers but only elevate the apparent thickness and thus the liquid absorption/permeation characteristics cannot be improved thereby. It has also been attempted to produce a high-bulkiness absorbent pulp by bulkily laminating pulp sheets by a dry method such as air-laying and binding these sheets together with the use of an appropriate binding agent. According to this method, an absorbent sheet, which has an extremely low density, a loose inter-fiber structure and a large absorption space in a dry state, can be obtained. When this absorbent paper absorbs a liquid, however, it shows very poor liquid diffusion characteristics in spite of its large absorption space. In addition, this sheet suffers from the same disadvantage as that of the fluff pulp, namely, it undergoes twisting/settling when pressure is applied to it in a wet state.
There have also been used cellulose-based nonwoven fabric absorbent sheets such as rayon spun bonded sheets. In a nonwoven fabric absorbent sheet composed of a single fiber, the liquid diffusion characteristics and the liquid absorption/permeation characteristics are mutually contradictory. The liquid diffusion characteristics have a tendency to be improved by reducing the fiber diameter. In this case, however, the inter-fiber distance is shortened and thus the liquid absorption/permeation characteristics are worsened. When the fiber diameter is enlarged, on the contrary, the liquid absorption/permeation characteristics are improved but the liquid diffusion characteristics are worsened. That is to say, the liquid absorption/permeation characteristics and liquid diffusion characteristics are contradictory to each other and it was therefore impossible to obtain an absorbent sheet satisfying both of these requirements, namely, high liquid diffusion characteristics and excellent liquid absorption/permeation characteristics.
JP-A-4-89053 has reported a prior technique for obtaining a super-thin absorbent article free from leakage (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). According to this patent, a super-thin absorbent article can be obtained by constructing an absorbent with the use of a combination of various absorbent sheets differing in absorption/permeation characteristics and diffusion characteristics from each other with high-absorbent polymers. However, this super-thin absorbent article has a disadvantage such that it is designed by taking the flow of bodily fluid in the absorbent article and the functions (temporary absorption/permeation/diffusion/retention of the bodily fluid) of each absorbent sheet into consideration and, as a result, the number of the absorbent sheets is increased and the structure of the absorbent becomes complicated. This disadvantage brings about undesirable results, namely, a decrease in productivity and an increase in production cost.
Accordingly, it is an object of the present invention to obtain an absorbent paper with a good performance, which has a large absorption space, good absorption/permeation characteristics and excellent diffusion characteristics, and to provide an absorbent article provided with an absorbent paper, which has an extremely high absorption capacity, suffers from little leakage, has a high thinness and can be comfortably fitted.